mitt romney and paul ryan will be kicking off a bus tour in ohio, to regain momentum in a state where they're trailing. but president obama will be at the united nations, telling the assembly of leaders that the united states will do what we must to prevent iran from obtaining a nuclear weapon. a senior administration official tells abc news this morning in new york city, in front of the united nations, president obama will reiterate the u.s. commitment to preventing iran from acquiring a nuclear weapon. he'll also call on leaders in the combustible muslim world to end the violence and declare the u.s. will not shrink from bringing to justice those who harm americans. sensing an opening, mitt romney has been assailing the president for calling the situation bumps in the road. >> bumps in the road -- we had an ambassador assassinated. and iran is that much closer to building a nuclear weapon. >> reporter: the white house says the president was referring broadly to transformations in the region. >> there is a certain rather desperate attempt to grasp at words and phrases here to find po